As stated in the Encyclopedia of Polymer Science and Technology, Volume 3, page 325 (1972), "Cellulose is a polyhydroxy compound and is therefore capable of reacting with such reagents as organic acids, anhydrides, and acid chlorides to form organic esters. Theoretically, (cellulose) esters of almost any organic acid can be prepared, . . . " For example, cellulose acetate, the most important commercial cellulose ester, has been conventionally prepared by treatment of cellulose pulps in batch-wise operations with acetic acid and acetic anhydride, catalyzed by a mineral acid such as sulfuric acid.
A detailed history of organic cellulose esters is provided on pages 325-354 of the above identified Polymer Encyclopedia volume.
Furthermore, Volume 4 of Kirk Othmer Encyclopedia of Chemical Technology, pages 632 to 637 (1970), sets forth additional background material on cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate, respectively.
The respective cellulose and cellulose triacetate molecules are pictured on page 329 of the previously described Polymer Encyclopedia article. In order to prepare a cellulose acetate for use in its main application, i.e., fibers for the textile industry, a product having an acetyl content of from about 37 to about 41% must be prepared. Another way of characterizing the cellulose acetate product is by using the term "degree of substitution" (DS). The degree of substitution (DS) is defined as the average number of hydroxyl groups substituted, of the three hydroxyl groups available for substitution in the anhydro glucose units. For example, so-called cellulose triacetate has an acetyl content of 43.5% and a degree of substitution of about 2.8-3.0.
Two types of acetylation reactions have been suggested for preparing cellulose esters. The first is homogeneous or fibrous esterification. In the homogeneous process, which is by far the major means by which cellulose acetate is produced commercially, an excess of acetic acid and acetic anhydride are employed to form cellulose triacetate having a DS of at least 2.8. The cellulose triacetate produced is in solution in the form of a dope, i.e., a viscous, usually clear, cellulose acetate solution, preferably free of fibers. In order to prepare the desired cellulose ester product having the requisite lower degree of substitution, the cellulose triacetate dope is hydrolyzed by increasing the water content by about 5 to 10%.
As described on pages 337-341 of the Encyclopedia of Polymer Science and Technology article cited above, the major commercial process for the preparation of cellulose acetate is the solution or homogeneous acetylation process. The most commonly used catalyst in this process is, of course, sulfuric acid. The Encyclopedia article goes on to state that the esterification reaction to produce the triester contemplates adding cellulose and acetic acid to an acetylation mixture where, after the cellulose has been swollen and activated, a small portion of the sulfuric acid catalyst is added to initiate cleavage of the cellulose chain. At this point, the mixture is cooled and cold acetic anhydride is added thereto, thus causing any water in the system to be reacted by the acetylation mechanism. The mixture is then further cooled and the acetylation reaction initiated by adding the remainder of the sulfuric acid catalyst. The reaction temperature is regulated to gradually increase to 90.degree.- 95.degree. F. during an interval of about 1.5 to 2 hours to produce the aforementioned cellulose triester dope. A 60 to 75% mixture of acetic acid and water is then added to terminate the acetylation reaction at the requisite viscosity by destroying the excess anhydride present in the system. This termination step may require about an hour to complete. If the triester is a desired product, the catalyst is then neutralized and removed. If, however, the hydrolyzed lower D.S. product is desired, such as secondary cellulose acetate, the sulfuric acid concentration is generally reduced to the desired level for conducting the reaction, the temperature is adjusted, and the batch is transferred to an hydrolysis vessel where the cellulose solution is allowed to hydrolyze at constant temperature until desired acetyl value, as previously discussed, is reached. The cellulose acetate is then recovered by various known techniques.
On page 340 of the Encyclopedia of Polymer Science and Technology description, a more detailed discussion of the intricacies of acetylation is provided. More specifically, in the previously described conventional cellulose acetate process, the acetic acid is employed as a solvent for the cellulose triester during the reaction, the acetic anhydride being the esterifying agent and, at the same time, reacting with any water formed during the esterification process. Critical to the formation of a uniform cellulose acetate product is a uniform distribution of the sulfuric acid catalyst with respect to the cellulose molecule. However, since the sulfation reaction between the cellulose and the sulfuric acid is much faster than the acetylation reaction, the sulfuric acid combines completely, but not necessary uniformly, with the cellulose immediately after the addition of the acetic anhydride. Therefore, control of the kinetics of both the sulfation and subsequent acetylation reactions, respectively, to produce a uniform cellulose triester product is difficult, at best. Accordingly, the prior art has provided means for chemically driving and controling the sulfation and acetylation reaction kinetics. In the aforementioned conventional cellulose acetate formation process, for example, acetic anhydride acts as the driving force for chemically controling the kinetics of the respective sulfation and acetylation reactions. This is accomplished by the use of an excessive amount of expensive acetic anhydride to form the cellulose triacetate product while meticulously controling the reaction parameters over an inordinately long time period. By employing this tedious, step-wise method, i.e., activation of the cellulose molecule with sulfuric acid followed by acetylation employing acetic anhydride, the requisite uniform cellulose triester dope will, hopefully, be produced. As stated on lines 14-16 on page 340, of the Polymer Encyclopedia article, "Proper correlation of the initial speed of reaction, maximum temperature, and total time of esterification are important in production control and in obtaining a fiber-free clear solution of cellulose triacetate in acetic acid."
The above peculiarities of the cellulose acetylation reaction are said to be due to several factors. First, all of the cellulose hydroxyl groups may not be available for reaction because crystallinity or insolubility of the cellulose hinders access of the reagent to the hydroxyl groups. Second, excessive amounts of degradive side reactions must cause cleavage of the cellulose chains resulting in undesirable, nonuniform products having unsatisfactory physical and chemical properties. In the past, the degradation reactions have been controled by lowering the temperature and allowing the acetylation reaction to continue for long periods of time. Third, the rates of esterification of the primary hydroxyl groups of the cellulose molecule, as compared with the secondary hydroxyl groups, are different. As shown by C. J. Malm et al. in the Journal of the American Chemical Society, Volume 75, pages 80-84 (1953), the uncatalyzed reactions of cellulose with acetic anhydride indicate that primary hydroxy groups reacted ten times as fast as the secondary. Furthermore, when the reactions were catalyzed with sulfuric acid, the primary hydroxyl groups reacted two and one-half times as fast. This is a further important reason as to why the cellulose acetate formation reaction cannot be readily controled.
The proposed heterogeneous formation of cellulose acetate is accomplished topichemically without dissolving the cellulose fibers. Furthermore, a product having an optimum degree of substitution for acetone solubility (2.2-2.6) can theoretically be produced by this process in a direct manner, without going to the cellulose triester, thereby further reducing the need for employment of large, excess amounts of acetic acid and acetic anhydride. Until now, however, an economical process for producing uniformly substituted, heterogeneous cellulose esters, preferably in a direct manner, has not been commercially successful.
Thus, cellulose acetate, as well as other higher acid esters, are still, for the most part, produced in batch-wise operations requiring considerable time, using relatively large amounts of excess anhydride. Thus, the above standard conventional procedure, as well as requiring a high capital investment owing to the need for extensive equipment to maintain the cellulose and reactants during the tedious formation process, also requires a high material cost owing to the necessity for using excessive amount of expensive organic anhydrides.
Various patents describe complex processes for making organic acid esters of cellulose. For example, U.S. Pat. No. 2,966,485 to Laughlin et al. relates to the production of cellulose esters employing a series of at least four successive reaction zones in an attempt to form uniform homogeneous product. In British Pat. Nos. 740,171 and 802,863 to Societe Rhodiaceta, tubular esterification zones and provided for conducting the requisite esterification reaction. In U.S. Pat. No. 2,778,820 to Clevy et al. and U.S. Pat. No. 2,854,446 to Robin et al., cellulose fibers, which have been previously beaten at low consistency, are employed as the cellulose feed stream for subsequent cellulose ester formation. Other patents, such as U.S. Pat. No. 3,525,734 to Rajon, describe complex processes for acetylation and/or hydrolysis in producing cellulose acetate including modified catalyst systems, the addition of stabilizers, or by providing other additional steps to an already lengthy formation procedure. Other systems, such as described in U.S. Pat. No. 3,273,807 to Wright, provide a process for premixing conditioning fluid, such as acetic acid, with cellulose fiber solids to facilitate the production of fluffed pulp, the respective fibers being individually coated with conditioning fluid. In this case, a refiner is used to perform the premixing function.